Presently, a bonding wire 20 to 50 μm or so in diameter and made of gold of a purity of 4N (four nine) (purity >99.99 mass %) is mainly used for connecting a pad of a semiconductor device with an outer lead. The ultrasonic bonding or welding method with thermocompression is generally used for bonding the gold bonding wire, and a general purpose bonding machine and a capillary jig through which the wire is fed for the connection work are required for the method. The leading end of the wire is melted by arc heat input to form a molten metal ball under surface tension and the ball is bonded by compression onto a pad of a semiconductor device heated to a temperature in a range of 150 to 300° C., and then the wire is directly bonded to an outer lead by ultrasonic bonding or welding with compression.
Semiconductor device packaging technologies have rapidly diversified lately in the aspects of structure, materials, pad connections, etc.: for instance, new packaging structures such as the ball grid array (BGA) method and the chip scale packaging (CSP) method, using a substrate and a polyimide tape, etc. have come to be employed in addition to the presently used quad flat packaging (QFP) method using a lead frame. These new packaging methods require a new bonding wire having an enhanced loop property, bondability, and productivity and usability properties. Among various wire connecting methods, in addition to the ball/wedge bonding which is presently the mainstream technology, the wedge/wedge bonding method suitable for narrower pitch bonding requires an improved bondability of the thin wire since the wire is directly bonded at two points.
The materials of the portions to which bonding wires are bonded are also changing. Cu, which is better suited for fine wiring, has come to be used, in addition to conventional Al alloys, for the wiring and pads of a silicon substrate. Besides this, the thickness of a pad film is becoming thinner and thinner, as the wiring becomes denser, which makes damage, such as cracking of the semiconductor substrate immediately beneath the pad film when the ball at the wire en is bonded onto it, a real danger. Further, in pursuit of downscaling and densification of semiconductor devices, a new type of semiconductor device hitherto considered difficult to produce wherein elements are formed beneath the pad films is being developed and, as a result, a reduction in the damage to the semiconductor devices caused by the ball bonding has become more and more important. For copying with these changes in the materials to which the bonding wire is bonded, higher bondability and bonding reliability are required of a bonding wire.
To respond to the higher integration and densification of semiconductor devices, the required connection properties of a gold bonding wire have become more widely varied; the most strongly required being (1) narrower pitch bonding, (2) the use of thinner wires, (3) increased number of pins and longer wires, (4) small ball bonding, (5) less damage to a chip at the bonded portions, etc.
In the resin molding process in which a high viscosity thermosetting epoxy resin is injected onto a semiconductor device at a high speed, for instance, there occurs a problem of a bonding wire deforming to contact an adjacent wire. The prevention of the wire deformation during the resin molding to the greatest possible extent is imperative as the narrower pitch bonding and the use of longer and thinner wires are becoming more widely practiced. Although an increase in wire strength is effective for controlling the wire deformation to some extend, it cannot be practically applied unless problems such as difficulty in loop control and a decrease in the bonding strength are solved.
For coping with these requirements, it is necessary for a bonding wire to satisfy the overall characteristics covering aspects such as the ease of the loop control during the bonding work, good bondability to the pads and leads, the prevention of wire deformation during the resin molding after wire bonding.
The addition of alloy elements has conventionally been the principal measures to increase the strength of the bonding wire. In the case of the gold bonding wire of high purity, which is most commonly used at present, however, the addition of alloy elements is limited to several tens of ppm. With this level of alloy element addition, while excellent loop control and bondability are achieved, sufficiently good results have not been obtained in aspects such as wire deformation and the strength of a heat affected zone (neck portion) at ball formation. High concentration alloy wires containing 1% or so of alloy elements in total have lately come to be used in some IC devices, but the effect to prevent the wires from deforming during the resin molding work is not sufficient and problems such as poor bondability with the leads have not been eliminated.
New bonding wire materials to substitute for gold have also been examined. However, with a copper wire, for example, it is difficult to obtain satisfactory results in increasing the wire strength and softening the ball portion and, for this reason, copper is not used for common LSIs. It is true that aluminum wires are used for the wedge/wedge bonding for ceramics packaging uses but aluminum cannot be used for general applications either, owing to problems such as surface corrosion after the resin molding, poor bondability when alloy elements are added in high concentrations, and so forth.
There have been proposals of bonding wires, the core wire and the periphery of which are composed of different metals (such a wire being hereinafter referred to as a “double layer bonding wire”), as a measure to increase the strength of the bonding wire. Japanese Unexamined Patent Publication No. S56-21354, for example, discloses a bonding wire wherein a core wire of Ag is coated with Au, Japanese Unexamined Patent Publication No. S59-155161 discloses another bonding wire having a core wire of a conductive metal and plated with Au on the surface, and Japanese Unexamined Patent Publication No. H4-79246 discloses another bonding wire having a core wire of Pt/Pt alloy and a periphery of Ag/Ag alloy. Additionally, as an example of a case of using materials containing the same elements in different concentrations for the two layers, Japanese Unexamined Patent Publication No. H6-252196 discloses a bonding wire having a core of an Au alloy containing Ca, Be, etc. and a periphery of another Au alloy containing the same alloy elements in lower concentrations. These bonding wires are expected to satisfy, in an overall manner, the properties suitable for any general purpose devices, which is difficult for a bonding wire consisting of a single material (hereinafter referred to as a “single layer bonding wire”), through the combination of different metals for the core wire and the periphery. Further, Japanese Unexamined Patent Publication No. H3-135040 discloses yet another bonding wire the surface of which is coated with an alloy element or a high concentration alloy and wherein the concentration of the alloy element is gradually and continuously changed from the periphery towards the center.
When the amount of an alloy element is increased for enhancing the strength of a conventional single layer bonding wire, bondability is deteriorated owing to factors such as the surface oxidation of the ball, the occurrence of defects such as cavity, or its wedge bondability is adversely affected by the hardening, oxidation, etc. of the wire surface, and these problems end up with a poor product yield of the final product. Besides, since electric resistance tends to rise with an increase in an alloy element, an excessive increase in electric resistance may cause apparent signal delay, especially in the case of an IC for high frequency use.
As a measure to solve the problems the single layer bonding wires cannot cope with, various combinations of materials have been proposed in relation to multi-layer bonding wires such as the double layer bonding wire composed of different materials in the core wire and the periphery. The double layer bonding wire, however, has not been brought to actual use and therefore few reports have so far been presented on the evaluation of its use in actual semiconductor devices. This is due to various difficulties. For example, the structure of the proposed double layer bonding wires to use different materials for the core wire and periphery requires very complicated production and quality control technologies to obtain a prescribed material ratio between the core wire and periphery in mass production. While a double layer bonding wire may be excellent in certain property aspects, the adhesion between its core wire and periphery tends to be poor, and thus it is difficult for the bonding wire to satisfy wide-ranged property requirements in an all-round manner.
The inventors of the present invention studied the characteristics of the double layer bonding wires and confirmed that there were various problems to be solved such as the poor adhesion between the core wire and the periphery, production and quality control technologies for securing a prescribed material ratio between the core wire and periphery, and the like.
In the case of double layer bonding wires so far proposed in which the core wire and periphery are composed mainly of different metals, problems caused by poor interface adhesion are often seen, since the different materials are simply in contact with each other. Although the adhesion is improved to some extent by wire drawing or similar work, it is difficult to obtain sufficiently strong adhesion only by working. Because different metal materials having different mechanical properties are drawn at the same time during the wire drawing work, unless the adhesion at the interface is good, there will be problems that the area ratio of the core wire and the periphery becomes longitudinally uneven during manufacturing and it becomes difficult to obtain prescribed product properties and that interface separation and cracking may occur during high speed drawing. Problems related to the interface separation of the double layer bonding wires will also occur during use when they are bent in complicated curves at a high speed and undergo heavy plastic deformation at the bonding.
Because different metal materials having different mechanical properties are drawn at the same time during the wire drawing work, it is difficult to obtain prescribed product properties as the area ratio of the core wire and periphery changes during manufacturing.
A double layer bonding wire can be manufactured by producing an ingot or a thick wire having a two-layer structure and drawing it into a final product diameter, or by forming a surface layer by plating, vapor deposition, or a similar method on the surface of a single layer wire drawn to near a final diameter. All of the methods, however, have problems related to manufacturing. In the former method it is difficult to obtain stable mechanical properties because of problems such as poor adhesion at the interface between the core wire and the periphery and the change in the area ratio of the core wire and the periphery from the initial ratio as a result of the wire drawing work in which different metal materials having different mechanical properties are drawn at the same time. The latter method also has problems such as insufficient interface adhesion between the core wire and the periphery and difficulty in obtaining a smooth wire surface, because the surface layer is formed at the final manufacturing stage.
Besides, while it is possible to increase the wire strength by the use of different materials composed mainly of different metals for the core wire and the periphery, this brings about a problem of bondability of the ball. More specifically, the materials of the core wire and periphery are mixed in the ball portion formed by melting the wire, forming a material containing alloy elements in high concentrations, which in turn makes the ball prone to hardening, surface oxidation, cavity, etc., causing problems such as the deterioration of bonding strength, cracks and other damage to the semiconductor substrate immediately beneath the portion where the ball is bonded. In the case that the core wire and periphery are mainly composed of different metals, for instance, the total content of the alloy elements in the molten ball often becomes as high as several percent to several tens of percent. In view of the above, it is important, in the actual use of a double layer bonding wire, to secure the bonding strength of the ball bonding and reduce the damage to the semiconductor substrate.
The problems related to the interface adhesion are likely to occur in the case of a double layer bonding wire in which the core wire and periphery are mainly composed of a metal common to both, as well as in the case of the core wire and periphery being mainly composed of different metals. Thus, the problems such as low productivity, interface separation at bending deformation and bonding, uneven wire properties, etc. remain. Although the use of the same metal as the main element can reduce the problems such as the damage to the semiconductor substrate at ball bonding seen in the case of the different metals, it is still difficult, by the same measure, to improve the strength and other mechanical properties to surpass those of the single layer bonding wire presently used, in spite of the double layer structure. For instance, the strength and elastic modulus of the wire, which are governed by the mixing rule determined by the sectional area ratio of the core wire and the periphery, will be influenced by the properties of the core wire and the periphery, which have similar properties owing to their being mainly composed of the common metal, and it is difficult for the strength and elastic modulus to surpass those of the core wire and periphery. Namely, in view of the fact that the strength and elastic modulus of a wire are often governed by the mixing rule determined by the sectional area ratio of the core wire and the periphery, it is difficult for either of them to surpass the material properties of either of the core wire and the periphery, whichever has the better strength or elastic modulus.
However, if an element or elements is/are added to the materials of the core wire and the periphery in a high concentration(s) for the purpose of increasing the strength, the problem of poor bondability of the ball and wedge occurs, as in the case of the single layer bonding wire. In order to cope with the narrower pitch bonding and the use of thinner and longer wires in the future, it is necessary to suppress the collapse of the loops and the deformation of the wires during resin molding. This, in turn, requires an improvement in the mechanical properties of the wire further to those of the double layer bonding wire so far proposed. The present inventors have confirmed that, among the mechanical properties to be improved, what has to be improved, to significantly reduce the wire deformation during the resin molding, is flexural rigidity.
In the case of a bonding wire the surface of which is coated with an alloy element or a high concentration alloy and in which the concentration of the alloy element is gradually and continuously changed from the periphery towards the center, it is difficult to achieve the same level of properties as the presently used single layer bonding wire, even when the former bonding wire is used together with a latest bonding apparatus and under the latest loop formation and bonding conditions. Even when the concentration of an alloy element is changed continuously near the surface of the wire, the loop shape may still become uneven and the linearity of the wire remain poor, because the measure is not enough for the wire to properly withstand the severe loop formation and friction with the inner wall of a capillary jig. In the above case, the thickness of the surface coating layer is several thousand Å or so, and the concentrations of the alloy elements change there. As a consequence, insufficient bonding strength often results from a significant deformation of the wire during the wedge bonding to the plated portion of a lead or a resin substrate and the existence of the thin coating layer at the connection interface between the wire and the plated portion. The layer having uneven element concentrations at the connection interface may also bring about inhomogeneous diffusion behavior at the connection interface. This may adversely affect the long-term reliability of the bonding.
From the above and also for coping with future packaging technologies of semiconductor devices, what is required of the material development of a bonding wire is not only to satisfy certain specific required characteristics but to improve the overall characteristics. Since this cannot be achieved only by means of composition design of the presently used single layer bonding wire, material selection of a double layer bonding wire or simply introducing a double layer structure, a bonding wire excellent in the loop controllability, having high strength and improved bondability and bonding strength, is required.
The object of the present invention is to provide a bonding wire for a semiconductor device having altogether: high strength and high flexural rigidity conforming to the narrower pitch bonding and the use of thinner and longer wires, excellent ball and wedge bondability, and excellent industrial mass-producibility and usability in high speed bonding; which have been the three requirements hardly satisfied by a conventional single or double layer bonding wire at the same time.